This invention relates to differential pressure filtration devices and, in particular, to a differential filtration device having an improved design which permits manufacturing with straight-pull molds and without mold mismatch flaws which arise from side action molds and can cause leakage.
Filtration devices employing differential pressure have been previously described in many patents. See, for example, Farr U.S. Pat. No. 3,481,477; Grover U.S. Pat. No. 3,693,804; Farr U.S. Pat. No. 3,969,250; Ahlstrand et al. U.S. Pat. No. 3,970,565; and Jaffe U.S. Pat. No. 4,035,150.
A typical prior art device (FIGS. 1 and 2) includes an outer container (a) which slidably receives a hollow plunger (b). A filter (c) is disposed near the end of the hollow plunger (b) and is retained there by a retainer ring (d) ultrasonically welded to the inside of the hollow plunger (b). Typically, an O-ring (e) is disposed in an annular groove (f) circumscribing the hollow plunger (b). The annular groove (f) is bounded and defined by shoulder portions (g) and (h) of increased diameter which prevent the O-ring (e) from moving with respect to the hollow plunger (b).
A major drawback of filtration devices of this type is the inability to manufacture them on straight-pull molds due to the annular groove (f) which makes it impossible to remove this part from a straight-pull mold. Consequently, previously known filtration devices have been manufactured in side action or two-part mold cavities divided longitudinally in half. As a result of two-part molds, mismatches known as parting lines (s) are inevitable along the seam joining the two halves. Even though mismatch flaws may only be on the order of a few thousandths of an inch, this can be enough to cause the O-ring (e) to seat improperly in the annular groove (f) and cause a leak. While flaws of this magnitude generally will not permit liquids to pass, they often will permit air to pass, causing a poor seal. A poor air seal compromises the pressurization of trapped air essential to good sample filtrate delivery in differential filtration devices.
In addition, heat generated by the ultrasonic welding of the retaining ring (d) to the end of the hollow plunger (b) can often damage the filter (c) which is adjacent thereto. Moreover, the pressure with which the ring (d) compresses the filter (c) cannot be adequately controlled. Insufficient or non-uniform pressure permits bypass leaks, while excessive pressure can damage the filter.